Run Lambda when a caller disconnects

I have a contact flow that transfers customers to an external number. There is a lambda that should execute after the call disconnects. I’m using the transfer to phone number block under terminate/transfer with resume flow enabled. If the agent disconnects first, the flow continues and executes the lambda. If the caller disconnects first, the lambda never executes.

Is it possible to trigger a lambda in the event a user disconnects? The only way I could see looking at documentation is to set up a firehose and responding to contract trace events. Am I overlooking something?

This seems unnecessarily complicated. Adding support for a Caller Hang-Up event in the transfer block seems like a really convenient solve to this problem.